CN115427487A

CN115427487A - Polypropylene resin expanded beads and molded article of polypropylene resin expanded beads

Info

Publication number: CN115427487A
Application number: CN202180012755.1A
Authority: CN
Inventors: 岛田智仁; 太田肇
Original assignee: JSP Corp
Current assignee: JSP Corp
Priority date: 2020-02-04
Filing date: 2021-01-21
Publication date: 2022-12-02
Anticipated expiration: 2041-01-21
Also published as: KR20220137936A; WO2021157369A1; US11732101B2; JPWO2021157369A1; CN115427487B; US20230074915A1; TW202140651A; EP4101628A4; EP4101628A1

Abstract

A foamed bead having a through-hole and including a foamed core layer defining the through-hole therein and composed of a resin composition containing two types of polypropylene-based resins having different melting points, and a cover layer covering the foamed core layer and composed of a polyolefin-based resin. The expanded beads provide a DSC curve in which an endothermic peak inherent to the resin composition and another endothermic peak on the higher temperature side thereof appear at a specific heat of fusion ratio.

Description

Polypropylene resin expanded beads and molded article of polypropylene resin expanded beads

Technical Field

The present invention relates to polypropylene-based resin expanded beads having through-holes and a polypropylene-based resin expanded bead molded article having interconnected pores and obtained by molding the expanded beads in a mold.

Background

Since a polypropylene-based resin expanded bead molded article having interconnected pores (hereinafter sometimes simply referred to as "expanded bead molded article having pores" or "expanded bead molded article") has excellent water permeability, air permeability and sound absorption properties, and also has excellent cushioning properties, it is used for drainage materials, wall materials of buildings, interior materials of automobiles, and the like. Further, in recent years, expanded bead molded articles having pores have been used as seat members for automobiles due to their excellent adhesion properties to polyurethane foams. Such an expanded molded article used as a seat member of an automobile may be, for example, an expanded molded article disclosed in patent document 1.

In recent years, expanded bead molded articles having pores have been required to have higher rigidity while maintaining the porosity thereof. As a method for obtaining an expanded bead molded article having excellent rigidity, it is conceivable to use a polypropylene-based resin having a high melting point. However, in-mold molding of polypropylene resin expanded beads having a high melting point requires the use of a high vapor pressure. Therefore, a problem arises that it is difficult to perform in-mold molding at a low-pressure vapor pressure.

To solve such a problem, patent document 2 discloses propylene resin expanded beads having a columnar core layer in an expanded state and a cover layer covering the expanded core layer. The core layer is composed of a resin in which a propylene-based resin having a low melting point and a propylene-based resin having a high melting point are mixed at a specific ratio, and the cover layer is composed of an olefin-based resin having a melting point lower than that of the core layer. With this constitution, the expanded beads are described as being capable of molding a foamed article of expanded beads having voids excellent in rigidity in a low-pressure mold.

Documents of the prior art

Patent document

Patent document 1 kokai JP2019-30597

Patent document 2, U.S. patent No. 10,487,188.

Disclosure of Invention

Problems to be solved by the invention

However, the expanded beads disclosed in patent document 2 involve a limitation of the vapor pressure range that allows in-mold molding of an expanded bead molded article having a high porosity. More specifically, with the expanded beads described in patent document 2, when the vapor pressure used in-mold molding is high, the porosity of the obtained expanded bead molded article may decrease.

In view of the above problems, an object of the present invention is to provide polypropylene-based resin expanded beads having through-holes, which are capable of molding expanded bead molded articles having voids and excellent rigidity, which allow in-mold molding using a low vapor pressure, which are capable of in-mold molding expanded bead molded articles having a high porosity even using a vapor pressure so high that a reduction in porosity would have conventionally been caused, and which allow in-mold molding with a wide range of pressures. The present invention also provides, as its object, a polypropylene-based resin expanded bead molded article obtained by in-mold molding of expanded beads and having excellent rigidity and high porosity.

Means for solving the problems

According to the present invention, there are provided expanded beads having through-holes and an expanded bead molded article having interconnected pores, as follows.

[1] A foamed bead having through-holes, the foamed bead including a foamed core layer defining the through-holes therein and including a polypropylene-based resin composition, and a cover layer covering the foamed core layer and including a polyolefin-based resin,

wherein the polypropylene-based resin composition comprises 70 to 97wt% of a polypropylene-based resin PP1 having a melting point higher than 140 ℃ and not higher than 150 ℃, and 3 to 30wt% of a polypropylene-based resin PP2 having a melting point not lower than 145 ℃ and not higher than 160 ℃, provided that the total amount of the resin PP1 and the resin PP2 is 100wt%,

wherein the difference between the melting point of the polypropylene-based resin PP2 and the melting point of the polypropylene-based resin PP 1[ (melting point of PP 2) - (melting point of PP 1) ] is not less than 5 ℃ and less than 15 ℃,

the expanded beads have a crystal structure such that when measured by thermal flow differential scanning calorimetry in which the expanded beads are heated from 30 ℃ to 200 ℃ at a heating rate of 10 ℃/min, a first DSC curve is provided,

wherein the first-order DSC curve has a main endothermic peak inherent to the polypropylene-based resin composition and a high-temperature side endothermic peak located on the higher temperature side of the main endothermic peak, wherein the high-temperature side endothermic peak has a heat of fusion of 12 to 20J/g, and

wherein a ratio of a heat of fusion of the main endothermic peak to a heat of fusion of the high-temperature side endothermic peak is 3.5 or more.

[2] The polypropylene-based resin expanded bead according to the above [1], wherein the melting point of the polypropylene-based resin PP1 is higher than 140 ℃ and not higher than 145 ℃, and the melting point of the polypropylene-based resin PP2 is not lower than 150 ℃ and not higher than 155 ℃.

[3] The polypropylene-based resin expanded bead according to the above [1] or [2], wherein the polypropylene-based resin PP2 has a Melt Flow Rate (MFR) of 2 to 18g/10min at 230 ℃ under a load of 2.16 kg.

[4] The polypropylene-based resin expanded bead according to any one of the above [1] to [3], wherein the polypropylene-based resin PP1 and the polypropylene-based resin PP2 are each a polypropylene-based resin obtained by polymerization using a Ziegler-Natta catalyst.

[5]According to the above [1]-[4]The polypropylene-based resin expanded beads according to any one of the preceding claims, wherein the expanded beads have a density of 15 to 50kg/cm ³ The bulk density of (2).

[6] The polypropylene-based resin expanded beads according to any one of the above [1] to [5], having an average outer diameter D [ mm ], an average pore diameter D [ mm ] of through-holes and an average wall thickness t [ mm ] defined as (D-D)/2,

wherein t is 0.8 to 2mm and t/d is 0.4 to 1.

[7] A polypropylene-based resin expanded bead molded article comprising a plurality of expanded beads according to any one of the above [1] to [6], the expanded beads being fusion-bonded to each other, the expanded bead molded article being formed with mutually communicating pores and having a porosity of 20% or more.

Effects of the invention

According to the present invention, there are provided polypropylene-based resin expanded beads which can be in-mold molded in a wider pressure range than before, and an expanded bead molded article having excellent rigidity and high porosity.

Drawings

Fig. 1 is a perspective view illustrating one embodiment of the expanded beads of the present invention.

Fig. 2 (a) is a schematic cross-sectional view showing an example of the foamed core layer of the present invention.

Fig. 2 (b) is a schematic cross-sectional view showing another example of the foam core layer.

Fig. 2 (c) is a schematic cross-sectional view showing still another example of the foam core layer.

Fig. 2 (d) is a schematic cross-sectional view showing still another example of the foamed core layer of the present invention.

Fig. 2 (e) is a schematic cross-sectional view showing still another example of the foam core layer.

Fig. 2 (f) is a schematic cross-sectional view showing still another example of the foam core layer.

Fig. 2 (g) is a schematic cross-sectional view showing still another example of the foamed core layer.

Fig. 2 (h) is a schematic sectional view showing still another example of the foam core layer.

Fig. 3 (a) is a schematic cross-sectional view showing still another example of the foam core layer.

Fig. 3 (b) is a schematic cross-sectional view showing still another example of the foam core layer.

Fig. 3 (c) is a schematic cross-sectional view showing still another example of the foam core layer.

Fig. 3 (d) is a schematic cross-sectional view showing still another example of the foam core layer.

Fig. 4 is a schematic view showing an example of the state of the surface of the expanded bead molded article of the present invention having interconnected pores.

Fig. 5 is a schematic diagram showing an example of a first-order DSC curve in heat-flow differential scanning calorimetry of the expanded beads of the present invention.

Detailed Description

The polypropylene-based resin expanded beads having through-holes and the polypropylene-based resin expanded bead molded article having interconnected pores (hereinafter, sometimes simply referred to as "expanded bead molded article" or "molded article") according to the present invention will be described in detail below. In the present specification, "a to B" indicating a numerical range is synonymous with "a or more and B or less" indicating a numerical range including a and B as endpoints of the numerical range. Furthermore, in the specification and claims, the singular form (a, an, the) includes the plural form unless the context clearly dictates otherwise. Thus, for example, "expanded beads" is intended to include "two or more expanded beads". The singular forms "a", "an" and "the" include plural referents unless the context clearly dictates otherwise.

The polypropylene resin expanded beads having through holes of the present invention (hereinafter, may be simply referred to as "expanded beads") are multilayer expanded beads each having a foam core layer and a cover layer covering the foam core layer. An intermediate layer may be provided between the foamed core layer and the cover layer, but it is preferable not to provide such an intermediate layer because it is desirable to reduce the thickness of the cover layer for the reasons described later. The expanded beads are preferably non-crosslinked for reasons of excellent recyclability, production efficiency, and the like.

Fig. 1 is a schematic view showing an example of polypropylene-based resin expanded beads having a tubular foam core layer and a covering layer covering the foam core layer. The expanded beads denoted by reference numeral 2 are generally columnar foams having through-holes 4. The expanded beads 2 include an expanded core layer 3 and a cover layer 5, the expanded core layer 3 defining through-holes 4 therein, the cover layer 5 covering the expanded core layer 3.

The cover layer 5 may or may not be foamed. For the purpose of achieving an increase in rigidity of the expanded bead molded article, however, it is preferable that the covering layer 5 is not expanded. As used herein, "is unfoamed" refers not only to a case where cells are not present at all in the covering layer (including a case where cells once formed are melted and broken so that the cells disappear), but also to a case where very fine cells are present in a small amount, that is, a case where substantially no foaming occurs. Further, "covering the foam core layer 3" does not mean that the entire foam core layer 3 should be covered; i.e. allowing a portion of the foamed core layer 3 to be exposed. Preferably, the entire outer peripheral surface on the side of the columnar foam core layer 3 is covered, because the melting property of the expanded beads can be more reliably improved. All or most of each end face of the foam core layer 3 in which the through-holes 4 are open is not covered with the cover layer 5.

Next, the resin constituting the foamed core layer 3 will be described.

The foamed core layer includes a polypropylene-based resin composition (a) (hereinafter, may be simply referred to as "resin composition (a)").

In the present specification, the polypropylene-based resin refers to a resin having, as a main constituent unit, a constituent unit derived from propylene. As used herein, the term "main constituent unit" means a content percentage of propylene component units in the polymer of more than 50wt%, preferably 80wt% or more, more preferably 90wt% or more.

The polypropylene-based resin may be a propylene homopolymer or a propylene-based copolymer containing a structural unit derived from propylene and other structural units. As the propylene-based copolymer, there may be mentioned a copolymer of propylene and ethylene or/and an α -olefin having 4 to 20 carbon atoms. Specific examples thereof include copolymers of propylene with one or more comonomers selected from ethylene, 1-butene, 1-pentene, 1-ethylene, 1-octene, 4-methyl-1-butene, 4-methyl-1-pentene, and the like. Further, the propylene-based copolymer may be a binary copolymer such as a propylene-ethylene random copolymer, a propylene-1-butene random copolymer, etc., or a ternary copolymer such as a propylene-ethylene-butene random copolymer, etc. Expanded beads comprising these random copolymers as a main component are excellent in secondary expansion (expansion) performance and also excellent in mechanical properties at the time of in-mold molding.

The total content of ethylene or/and comonomer components such as α -olefin having 4 to 20 carbon atoms in the propylene-based copolymer is preferably 25wt% or less, and more preferably 10wt% or less.

The polypropylene-based resin composition (a) constituting the foamed core layer of the present invention contains, as its main components, a polypropylene-based resin PP1 (hereinafter sometimes simply referred to as "resin PP1" or "PP 1") and a polypropylene-based resin PP2 (hereinafter sometimes simply referred to as "resin PP2" or "PP 2") satisfying the following requirements (i) and (ii):

(i) The polypropylene-based resin composition (a) comprises 70 to 97wt% of a propylene-based resin PP1 having a melting point MP1, and 3 to 30wt% of a polypropylene-based resin PP2 having a melting point MP2, the melting point MP1 being higher than 140 ℃ and not higher than 150 ℃, the melting point MP2 being not lower than 145 ℃ and not higher than 160 ℃ provided that the total amount of the resin PP1 and the resin PP2 is 100wt%,

(ii) A difference (MP 2-MP 1) between the melting point MP2 of the resin PP2 and the melting point MP1 of the resin PP1 is not less than 5 ℃ and less than 15 ℃; in other words, MP1 and MP2 satisfy the following relationship:

5℃≤MP2-MP1<15℃。

the meanings of the requirements (i) and (ii) and the effects thereof will be described in detail below.

As used herein, the term "comprising as its main components the resin PP1 and the resin PP2" is intended to mean that the total amount of PP1 and PP2 in the resin composition (a) exceeds 50wt%. The total amount of the resins PP1 and PP2 in the resin composition (a) is preferably 80wt% or more, more preferably 90wt% or more.

Among the above polypropylene-based resins, as the resins PP1 and PP2, the following polypropylene-based resins are more preferable.

The resin PP1 is preferably a propylene-based random copolymer, more preferably a propylene-ethylene random copolymer. In view of the melting point and foamability of the resin PP1, the total content of the comonomer components in the resin PP1 is preferably 0.5% by weight or more, more preferably 1.0% by weight or more, still more preferably 1.5% by weight or more. For the same reason, the total content of the comonomer component in the resin PP1 is preferably 10% by weight or less, more preferably 8.0% by weight or less, still more preferably 5.0% by weight or less.

The resin PP2 is preferably a propylene-based random copolymer for reasons of its excellent compatibility with the resin PP1 and the expected improvement in the mechanical properties and secondary expandability of the expanded beads obtained. Among them, a random copolymer of propylene and ethylene or/and 1-butene is more preferable. The total content of the comonomer component in the resin PP2 is preferably 0.3% by weight or more, more preferably 0.5% by weight or more, still more preferably 0.8% by weight or more in view of the melting point, mechanical properties, etc. of the resin PP 2. For the same reason, the total content of the comonomer component in the resin PP2 is preferably 5.0% by weight or less, more preferably 3.0% by weight or less, still more preferably 1.0% by weight or less.

In order to more reliably achieve the effect of the object of the present invention of being able to mold a molded article having excellent rigidity and high porosity in a wide pressure range, it is more preferable that the total content of the comonomer component in the resin PP2 is lower than that in the resin PP 1.

It is preferable that the resins PP1 and PP2 used in the present invention are polypropylene-based resins obtained by polymerization in the presence of a Ziegler-Natta polymerization catalyst. Polypropylene resins produced using ziegler-natta polymerization catalysts tend to have broader molecular weight distributions than polypropylene resins produced using metallocene catalysts. Therefore, the expanded beads made of the polypropylene resin polymerized using the ziegler-natta catalyst are superior in secondary expansion performance compared to the expanded beads made of the polypropylene resin polymerized using the metallocene polymerization catalyst, and show superior secondary expansion performance at a vapor pressure in a wide range from low pressure to high pressure.

The ziegler-natta-based polymerization catalyst is an organometallic complex containing titanium, aluminum, magnesium, or the like as a core element and partially or completely modified with an alkyl group.

The polypropylene-based resin composition (a) may contain other resins, polymers such as rubbers and elastomers, additives, and the like to the extent that the effects of the present invention are achieved.

As the other resin component, there can be mentioned, for example, vinyl resins such as high density polyethylene, low density polyethylene, linear ultra low density polyethylene, ethylene-vinyl acetate copolymer, ethylene-acrylic acid copolymer and ethylene-methacrylic acid copolymer, styrenic resins such as polystyrene and styrene-maleic anhydride copolymer, and polyamide resins. The rubber and elastomer may be rubbers such as ethylene-propylene rubber, ethylene-1-butene rubber, propylene-1-butene rubber, ethylene-propylene-diene rubber, isoprene rubber, chloroprene rubber, nitrile rubber and the like and thermoplastic elastomers such as styrene-butadiene-styrene block copolymer, styrene-isoprene-styrene block copolymer, hydrogenated product of styrene-butadiene-styrene block copolymer, hydrogenated product of styrene-isoprene-styrene block copolymer, and the like.

These polymers may be used in combination of two or more. When the resin composition (a) contains such other polymer components, the total amount thereof is preferably 20 parts by weight or less, preferably 15 parts by weight or less, more preferably 10 parts by weight or less, and particularly preferably 5 parts by weight or less, based on 100 parts by weight of the entire resin composition (a).

Examples of the additives include various types of additives such as cell control agents, e.g., zinc borate, antistatic agents, flame retardants, conductivity imparting agents, lubricants, antioxidants, ultraviolet absorbers, metal deactivators, pigments, dyes, crystallization nucleating agents, and inorganic fillers. These may be incorporated into the resin composition (a) as required. Strictly speaking, the amount of the additive varies depending on the purpose of addition, but it is preferably 0.5 to 25 parts by weight, more preferably 1 to 15 parts by weight, still more preferably 3 to 15 parts by weight, based on 100 parts by weight of the sum of the resin PP1 and the resin PP 2.

Because of the excellent balance among foamability, moldability, mechanical properties and the like, it is preferred that the Melt Flow Rate (MFR) of the resin PP1 is 1g/10min or more, more preferably 3g/10min or more, still more preferably 6g/10min or more, and preferably 30g/10min or less, more preferably 20g/10min or less, still more preferably 15g/10min or less.

Because of the excellent effect of improving the foamability under a wide range of pressure and the excellent effect of improving the mechanical physical properties, it is preferable that the Melt Flow Rate (MFR) of the resin PP2 is 2g/10min or more, more preferably 3g/10min or more, still more preferably 5g/10min or more, and preferably 18g/10min or less, more preferably 15g/10min or less, still more preferably 12g/10min or less.

As used herein, the MFR of the polypropylene-based resins PP1 and PP2 and the polypropylene-based resin composition (a) is a value as measured under test condition M (temperature of 230 ℃, load of 2.16 kg) of JIS K7210 (2014), while the MFR of the polyethylene-based resin is a value as measured under test condition D (temperature of 190 ℃, load of 2.16 kg) of JIS K7210 (2014).

The expanded beads having through-holes according to the present invention have a specific crystal structure, which satisfies the following requirements (1) and (2).

That is, the expanded beads have a crystal structure such that a first DSC curve is provided when measured by thermal flow differential scanning calorimetry in which the expanded beads are heated from 30 ℃ to 200 ℃ at a heating rate of 10 ℃/min. The first DSC curve has a main endothermic peak inherent to the polypropylene-based resin composition (a) and a high-temperature side endothermic peak located on the higher temperature side of the main endothermic peak, wherein:

(1) The heat of fusion DeltaHh (high-temperature peak calorific value) of the high-temperature side endothermic peak is 12 to 20J/g, and

(2) The ratio of the heat of fusion Δ Hm of the intrinsic main endothermic peak (intrinsic peak calorific value) to the heat of fusion Δ Hh of the high-temperature side endothermic peak, Δ Hm/Δ Hh, is 3.5 or more.

The expanded beads of the present invention, which comprise the resin composition (a) comprising the polypropylene-based resin PP1 and the polypropylene-based resin PP2 as main components and also have a crystal structure satisfying the essential conditions (1) and (2), exhibit excellent melting properties and secondary expansion properties and allow molding of an excellent expanded bead molded article even in a low vapor pressure mold. Further, even when in-mold molding is performed at such a high vapor pressure that causes a decrease in porosity using a conventional method, it is possible to suppress the decrease in porosity. Moreover, the obtained expanded bead molded article has excellent rigidity.

The meanings of the requirements (1) and (2) and the effects thereof will be described in detail below.

Next, the meanings of the requirements (i) and (ii) of the expanded beads and the effects thereof will be described in detail.

The polypropylene-based resin composition (a) constituting the foamed core layer of the present invention is (i) a mixed resin containing, as a main component, a low-melting resin PP1 having a melting point MP1 of more than 140 ℃ and 150 ℃ or less, and, as an auxiliary component, a high-melting resin having a melting point MP2 of 145 ℃ or more and 160 ℃ or less. Further, (ii) the difference (MP 2) - (MP 1) between the melting point MP2 of the resin PP2 and the melting point MP1 of the resin PP1 is 5 ℃ or more and less than 15 ℃.

Since the resin composition (a) constituting the expanded beads of the present invention contains the resin PP2 having a high melting point as an auxiliary component, the obtained expanded bead molded article has high rigidity according to, for example, the requirements for automotive seat members.

Further, since the resin composition (a) contains the resin PP1 having a low melting point as a main component, the expanded beads of the present invention allow in-mold molding at a low vapor pressure. In addition, although the expanded beads contain, as an auxiliary component, the resin PP2 having a melting point higher than that of the resin PP1, it is possible to obtain such an effect that the in-mold molding can be performed at a vapor pressure almost the same as that required for the in-mold molding with the resin PP1 itself or at a vapor pressure lower than such required.

The reason is considered as follows. In the expanded bead of the present invention, the foamed core layer is composed of a mixed resin in which a low-melting resin PP1 and a resin PP2 having a melting point higher than that of the resin PP1 are mixed in a specific ratio such that the ratio Δ Hm/Δ Hh of the inherent peak heating value Δ Hm to the high-temperature peak heating value Δ Hh in the first DSC curve tends to become large. That is, low potential crystals showing a peak on the low temperature side are present in the expanded beads of the present invention in a large amount and can contribute to the expansion thereof during the low vapor pressure in-mold molding. As a result, the expanded beads are considered to exhibit improved secondary expansion properties.

Because, in the expanded beads of the present invention, the low-melting resin PP1 having a melting point within the above range is mixed with the high-melting resin PP2, for example, it becomes easy to set the expansion temperature high. It is thus inferred that, for this reason, expanded beads having a crystal structure showing a large proportion of the calorific value of the main endothermic peak can be easily obtained. The crystal structure and setting of the foaming temperature to be high will be described below.

Further, with the expanded beads of the present invention, even when in-mold molding is performed at a high vapor pressure, a decrease in porosity of the obtained expanded bead molded article is suppressed. As a result, an effect is obtained in which the range of vapor pressure in which a molded article having a high porosity is obtainable is widened toward the high vapor pressure side. The reason why the decrease in the porosity of the expanded bead molded article is suppressed even when the expanded beads are molded in the high-pressure mold is considered as follows.

It is generally known that the porosity of multilayer expanded bead moldings decreases with increasing molding pressure. The reason for this is as follows. In the low vapor pressure in-mold molding, the cover layers are melted and melt-bonded before the completion of the secondary foaming of the core layer (while sufficiently maintaining the pore diameter of the through-holes), so that a molded article having a high porosity is easily obtainable. In the in-mold molding with a high vapor pressure, on the other hand, the secondary foaming of the core layer occurs and the melt bonding is performed in a state in which the through-holes are crushed. For this reason, the porosity tends to decrease.

In the expanded bead of the present invention, since the foamed core layer is composed of a mixed resin in which a resin PP1 having a specific melting point range and a resin PP2 are mixed in a specific ratio, since the resin PP2 having a high melting point is used as an auxiliary component, and since the polypropylene-based resin having a high melting point higher than 140 ℃ but not higher than 150 ℃ is also used as the resin PP1 having a lower melting point as a main component, the melting point of the entire core layer can be adjusted to a high level. It is considered that, due to the above-described results, a molded article having a high porosity can be in-mold molded in a wide vapor pressure range including from a low vapor pressure to a high vapor pressure.

The melting point MP1 of the polypropylene resin PP1 is higher than 140 ℃ and not higher than 150 ℃ or lower. When the melting point MP1 exceeds 150 ℃, there is a possibility that: the vapor pressure required for in-mold molding of the expanded beads increases. For this reason, the melting point MP1 is preferably 148 ℃ or lower, more preferably 146 ℃ or lower, still more preferably 145 ℃ or lower, particularly preferably 144 ℃ or lower. When the melting point MP1 is 140 ℃ or lower, if the vapor pressure during in-mold molding increases, there is a possibility that: the porosity of the molded article decreases. For this reason, the melting point MP1 is preferably 141 ℃ or higher, more preferably 142 ℃ or higher.

The melting point MP2 of the polypropylene resin PP2 is 145 ℃ or more and 160 ℃ or less. When the melting point MP2 exceeds 160 ℃, the adjustment of the high-temperature peak heating value of the expanded beads in the range of the requirement (1) is difficult, so that there is a possibility that: moldability of the expanded beads and rigidity of the resulting molded article decrease. For this reason, the melting point MP2 is preferably 158 ℃ or lower, more preferably 156 ℃ or lower, still more preferably 155 ℃ or lower, particularly preferably 153 ℃ or lower. When the melting point MP2 is less than 145 ℃, there is a possibility that: the rigidity of the molded article obtained is lowered. Further, it becomes difficult to control the ratio Δ Hm/Δ Hh of the intrinsic peak heating value Δ Hm of the expanded beads to the high-temperature peak heating value Δ Hh, so that there is a possibility that: the effect of reducing the molding pressure cannot be sufficiently obtained. For the above reasons, the melting point MP2 is preferably 146 ℃ or higher, more preferably 148 ℃ or higher, still more preferably 150 ℃ or higher, particularly preferably 151 ℃ or higher.

The difference (MP 2-MP 1) between the melting point MP2 of the resin PP2 and the melting point MP1 of the resin PP1 is 5 ℃ or more and less than 15 ℃. When the difference in melting points is too small, there is a possibility that: the range of vapor pressures that allow the formation of molded articles with high porosity is narrowed. For this reason, the melting point difference (MP 2-MP 1) is preferably 6 ℃ or more, more preferably 8 ℃ or more. When the difference in melting point is too large, it is difficult to control the foamability, so that there is a possibility that: the object and effect of the present application cannot be achieved. For this reason, the melting point difference (MP 2-MP 1) is preferably 13 ℃ or less, more preferably 12 ℃ or less.

The melting points MP1 and MP2 can be measured according to JIS K7121 (2012). More specifically, a resin sheet used as a sample was heated from 30 ℃ to 200 ℃ at a heating rate of 10 ℃/min. Thereafter, the temperature was decreased to 30 ℃ at a cooling rate of 10 ℃/min, and then increased again from 30 ℃ to 200 ℃ at a heating rate of 10 ℃/min to obtain a DSC curve. The temperature at the peak of the melting peak in the obtained DSC curve represents the melting point. When two or more melting peaks occur, the temperature of the apex of the melting peak having the largest area represents the melting point.

The content of the resin PP1 in the resin composition (a) is 70 to 97% by weight, and the content of the resin PP2 is 3 to 30% by weight. Here, the total amount of the resin PP1 and the resin PP2 is 100wt%. When the contents of the resin PP1 and the resin PP2 are within the above-mentioned ranges, an expanded bead molded article having high porosity and excellent rigidity can be obtained by in-mold molding in a wide range of vapor pressure.

When the content of the resin PP2 is too small, there is a possibility that: the desired effect of improving rigidity is not obtained. On the other hand, when the content of the resin PP2 is excessively large, the performance of the resin PP2 having a high melting point is substantially reflected, so that there is a possibility that: moldability on the low vapor pressure side is lowered.

For the above reasons, the lower limit of the content of the resin PP1 is preferably 75wt%, more preferably 80wt%, still more preferably 85wt%. The upper limit thereof is preferably 95wt%, more preferably 93wt%, still more preferably 90wt%. On the other hand, the lower limit of the content of the resin PP2 is preferably 5wt%, more preferably 7wt%, still more preferably 10wt%, and the upper limit thereof is preferably 25wt%, more preferably 20wt%, still more preferably 15wt%.

The DSC curve of the expanded beads of the present invention will be described below. The expanded beads obtained under the above-mentioned specific DSC measurement conditions have a primary DSC curve having a main endothermic peak inherent to the polypropylene-based resin composition (a) and a high-temperature side endothermic peak located on the higher-temperature side of the main endothermic peak, and further satisfy the following requirements (1) and (2):

(1) The heat of fusion Δ Hh of the high-temperature side endothermic peak is 12 to 20J/g; and

(2) The ratio of the heat of fusion of the main endothermic peak,. DELTA.Hm, to the heat of fusion of the high-temperature side endothermic peak,. DELTA.Hh, Δ Hm/Δ Hh, is 3.5 or more.

When the high-temperature peak calorific value Δ Hh is within the above range, the expanded beads have excellent moldability. Further, the rigidity of the expanded bead molded article obtained is improved. For the above reasons, the calorific value Δ Hh is preferably 13 to 18J/g.

When the ratio Δ Hm/Δ Hh is 3.5 or more, crystals (low potential crystals) showing a peak on the low temperature side increase, so that the secondary foaming property at the time of low vapor pressure in-mold molding is particularly improved.

For this reason, the ratio Δ Hm/Δ Hh is preferably 3.6 or more, more preferably 3.7 or more. The upper limit of the ratio Δ Hm/Δ Hh is usually 4.2. The ratio Δ Hm/Δ Hh can be controlled by adjusting the foaming temperature and blowing agent injection pressure at the time of producing expanded beads, or the temperature and holding time in the crystallization step, and the like. In the expanded beads of the present invention, since the low-melting resin PP1 and the high-melting resin PP2 having melting points within the range defined in the above-mentioned (i) are mixed, for example, the expansion temperature is set to be high to be easy, so that expanded beads having a crystal structure with a large Δ Hm/Δ Hh ratio can be easily obtained.

The high-temperature peak heating value Δ Hh and the intrinsic peak heating value Δ Hm (low-temperature peak heating value) of the expanded beads were measured by the following measurement methods based on JIS k7122 (2012). Expanded beads in an amount of 1 to 3mg were sampled and heated from 30 to 200 ℃ at 10 ℃/min with a thermal flow differential scanning calorimetry apparatus and measured. An example of a first DSC curve obtained by such a measurement is shown in figure 6.

In the DSC curve of fig. 6, a main endothermic peak a (intrinsic peak a) appears on the low temperature side, and a high temperature peak B appears on the high temperature side of the intrinsic peak a. The amounts of heat of the intrinsic peak a and the high-temperature peak B correspond to the areas of the respective peak regions, which can be specifically determined as follows.

The expanded beads are plotted on a line (α - β) connecting a point α on the DSC curve at 80 ℃ and a point β on the DSC curve at the final temperature T of melting. The final temperature T of melting is a crossing point at which the DSC curve on the high temperature side of the high temperature peak B satisfies the baseline. Next, a straight line is drawn parallel to the ordinate and passing through a point γ on the DSC curve of the bottom of the concave portion between the intrinsic peak a and the high-temperature peak B. This point, at which this line intersects the straight line (α - β), is called σ.

The area of the high-temperature peak B is an area bounded by a curve, a line segment (σ - β), and a line segment (γ - σ) of the high-temperature peak B of the DSC curve, and corresponds to the high-temperature peak calorific value Δ Hh. The area of the intrinsic peak is an area bounded by a curve of the resin intrinsic peak a of the DSC curve, a line segment (α - β), and a line segment (γ - σ), and corresponds to the intrinsic peak calorific value Δ Hm.

The high temperature peak was observed in the first DSC curve obtained as described above in heating a sample of expanded beads from 30 ℃ to 200 ℃ at 10 ℃/min, but not in the second DSC curve obtained by subsequently decreasing the temperature from 200 ℃ to 30 ℃ at 10 ℃/min and increasing the temperature to 200 ℃ again at 10 ℃/min after the first DSC measurement. Thus, by performing a second DSC measurement after the first DSC measurement to obtain a second DSC curve, it can be easily determined whether there is a high temperature peak in the first DSC curve. That is, the endothermic peak in the first-order DSC curve that appears in the first-order DSC curve but does not exist in the second-order DSC curve is considered to be a high-temperature peak.

The heat of fusion of the polypropylene-based resin PP1 is preferably 30 to 100J/g, more preferably 40 to 80J/g, still more preferably 50 to 75J/g, in order to further improve the expandability, moldability, mechanical strength and the like.

The heat of fusion of the resin PP2 is preferably higher than that of the resin PP 1. When the heat of fusion of the resin PP2 is higher than that of the resin PP1, the expanded beads are considered to be capable of being in-mold molded with a lower vapor pressure. For this reason, the heat of fusion of the resin PP2 is preferably 40 to 120J/g, more preferably 60 to 100J/g, still more preferably 75 to 90J/g.

Using a heat flux differential scanning calorimeter, using a resin sheet as a sample, based on JIS k7122:2012, the heat of fusion of the resins PP1 and PP2 can be measured. When a plurality of melting peaks appear on the DSC curve, the total area of the plurality of melting peaks represents the heat of fusion.

It is preferred that the resin PP1 has a flexural modulus of 600 to 1200MPa, more preferably 800 to 1,000MPa, for the reason of an excellent balance between expandability, moldability, mechanical properties and the like.

The flexural modulus of the resin PP2 is preferably 1,000 to 1,800MPa, more preferably 1,200 to 1,500MPa, for the reason of excellent effects of improving mechanical properties and the like.

The flexural modulus of the resin PP1 and the resin PP2 can be measured according to JIS k7171 (2008).

The covering layer of the expanded beads will be described below.

The cover layer is a layer covering the core layer and is composed of a polyolefin resin (b).

The polyolefin-based resin (b) means a resin containing, as a main constituent unit, a constituent unit derived from ethylene or an α -olefin such as propylene and 1-butene. Here, the main constituent unit means a constituent unit having a content of more than 50% by weight, preferably more than 80% by weight, in the polymer.

As examples of the polyolefin-based resin (b), the following (b 1), (b 2), and (b 3) may be mentioned.

(b1) Mention may be made of homopolymers of ethylene or of alpha-olefins, such as ethylene homopolymers and propylene homopolymers.

(b2) Mention may be made of copolymers of two or more types of monomer components selected from ethylene and alpha-olefins. The copolymer is preferably a copolymer containing at least one of a constituent unit derived from ethylene and a constituent unit derived from propylene. Examples of the ethylene copolymer include ethylene-1-pentene copolymer, ethylene-1-hexene copolymer and ethylene-4-methyl-1-pentene copolymer. Examples of the propylene copolymer include a propylene-ethylene copolymer, a propylene-1-butene copolymer and a propylene-ethylene-1-butene copolymer. These copolymers may be block copolymers, random copolymers or graft copolymers.

(b3) Mention may be made of copolymers of ethylene or/and of alpha-olefins and other monomeric components, such as styrene. The copolymer is preferably a copolymer whose constituent units derived from ethylene or/and an α -olefin are constituent units derived from ethylene or/and constituent units derived from propylene. For example, ethylene-styrene copolymers, ethylene-vinyl acetate copolymers, ethylene-methyl methacrylate copolymers and ethylene-methacrylic acid copolymers may be mentioned.

As the resin constituting the cover layer in the present invention, a polyethylene-based resin is preferable because of excellent melt bondability of the expanded beads. The polyethylene-based resin refers to a polymer or copolymer of the above (b 1) to (b 3), in which a constituent unit derived from ethylene is a main structural unit thereof. First, the copolymer of (b 2) is preferable. In particular, linear low density polyethylene and linear ultra low density polyethylene are more preferred.

The polyolefin-based resin (b) may be crystalline or amorphous. Whether the resin (b) is crystalline or amorphous can be determined from a DSC curve obtained by heat flow differential scanning calorimetry in which the resin (b) is used as a sample. In the case of crystalline resins, an endothermic peak appears on the DSC curve, whereas in the case of amorphous resins, no endothermic peak appears on the DSC curve.

When the polyolefin-based resin (b) is a crystalline polyolefin-based resin, it is preferable that the resin (b) has a melting point (TmB) lower than the melting point (TmA) of the polypropylene-based resin composition (a) and a difference [ TmA-TmB ] between the melting point (TmA) and the melting point (TmB) is more than 0 ℃ and not more than 80 ℃. When this condition is satisfied, the expanded beads have excellent melt-bonding properties. For this reason, the difference [ TmA-TmB ] is more preferably 5 ℃ or more and 60 ℃ or less, still more preferably 7 ℃ or more and 50 ℃ or less, particularly preferably 10 ℃ or more and 40 ℃ or less.

When the olefin-based resin (b) is an amorphous polyolefin resin, it is preferable that the resin (b) has a softening point (TsB) lower than the melting point (TmA) of the polypropylene resin composition (a) and the difference [ TmA-TsB ] between the melting point (TmA) and the softening point (TsB) is more than 0 ℃ and 100 ℃ or less. When this condition is satisfied, the expanded beads have excellent melt-bonding properties. For this reason, the difference [ TmA-TsB ] is more preferably 10 ℃ or higher and 80 ℃ or lower, still more preferably 15 ℃ or higher and 75 ℃ or lower, particularly preferably 20 ℃ or higher and 70 ℃.

The resin (b) can be polymerized using various polymerization catalysts. Examples of the polymerization catalyst include ziegler-natta-based polymerization catalysts and metallocene-based polymerization catalysts. Among these polymerization catalysts, metallocene-based polymerization catalysts are preferred. When a metallocene-based polymerization catalyst is used, an olefin-based resin having more excellent melting properties and a low melting point or a low softening point can be obtained.

To the extent that the effect of the present invention is achieved, the resin (b) constituting the covering layer of the expanded beads of the present invention may contain other polymer components that are allowed to be contained in the above-mentioned resin composition (a).

The total amount of such other polymer components in the covering layer is preferably about 20 parts by weight or less, more preferably 15 parts by weight or less, still more preferably 10 parts by weight or less, particularly preferably 5 parts by weight or less, based on 100 parts by weight of the resin (b).

In addition, the resin (b) may contain various additives. Examples of the additives include those similar to those used in the resin (a). The amount of the additive varies depending on the purpose of addition, but is preferably about 25 parts by weight or less, more preferably 20 parts by weight or less, still more preferably 15 parts by weight or less, and particularly preferably 8 parts by weight or less. The resin (b) may contain such an additive without suppressing the foamability.

It is preferred that the weight ratio (wt%) of foamed core layer to cover layer is 99.5:0.5 to 75, more preferably 98 to 80, still more preferably 96. When the weight ratio between the foam core layer and the cover layer is within the above range, the effect of improving melt bonding of the cover layer is further improved, and the rigidity of the obtained expanded bead molded article becomes more excellent.

In the expanded bead of the present invention, the thickness of the covering layer is preferably 1 to 50 micrometers, more preferably 2 to 20 micrometers, still more preferably 3 to 10 micrometers. When the thickness of the covering layer is within the above range, melt bondability at the time of in-mold molding is improved, and the rigidity of the obtained expanded bead molded article is further improved.

Next, the shape and size of the expanded beads of the present invention will be described.

As shown in fig. 1, the expanded bead 2 of the present invention has a foam core layer 3 and a cover layer 5, the foam core layer 3 has through holes 4, and the cover layer 5 covers the foam core layer 3. The overall shape is generally cylindrical. The foam core layer 3 may be embodied in various forms 3a to 3m, for example, as shown in fig. 2 (a) to 2 (h) and fig. 3 (a) to 3 (d). The foam core layer 3 is generally a foam core layer 3a having a circular cross section, as shown in fig. 2 (a). If necessary, a core layer 3b having a triangular cross section as shown in fig. 2 (b), a core layer 3c having a hexagonal cross section as shown in fig. 2 (c), a core layer 3d having a bisected circular cross section as shown in fig. 2 (d), core layers 3e and 3f having cross sections in which a plurality of circles are merged as shown in fig. 2 (e) and 2 (f), a core layer 3g having a circular cross section with a partially cut-off portion as shown in fig. 2 (g), and a core layer 3h having a rectangular cross section with a partially cut-off portion as shown in fig. 2 (h) may be employed. It is further possible to employ a core layer 3i having a cross section with a shape in which three limb portions e extend from a circular circumference as shown in fig. 3 (a), a core layer 3j having a cross section in which three limb portions e extend from respective sides of a triangle as shown in fig. 3 (b), a core layer 3k having a cross section in which six limb portions e extend from a circular circumference as shown in fig. 3 (c), and a core layer 3m having a cross section in which a total of six limb portions extend from the apex and respective sides of a triangle as shown in fig. 3 (d). The cross-sectional shape of the core layer 3 is not limited to the above-described cross-sectional shape, and may be an indefinite cross-sectional shape. Similarly, the cross-sectional shape of the through-hole 4 of the foam core layer 3 is generally circular, but has various shapes as needed, as shown in fig. 2 (a) to 2 (h) and 3 (a) to 3 (d).

It is preferable that the average pore diameter d of the through-holes of the expanded beads of the present invention is 1 to 3 mm. When the average pore diameter d is within this range, it becomes easier to adjust the porosity of the expanded bead molded article to a desired range. For this reason, the average pore diameter d is more preferably 1.2 to 2.5 mm.

It is also preferred that the average outer diameter D of the expanded beads is preferably 1.5 to 7 mm. When the average outer diameter D is within this range, excellent filling properties are obtainable, so that a superior expanded bead molded article can be obtained. For this reason, the average outer diameter D of the expanded beads is more preferably 2 to 6 mm, still more preferably 3 to 5 mm.

The average pore diameter d of the through-holes of the expanded beads was determined as follows. First, the expanded beads are cut perpendicular to the penetrating direction of the through-holes at the position where the area of the cut surface is the largest. A photograph of the obtained cut surface of the expanded beads was taken. The area of the through-hole portion in the photograph (i.e., the cross-sectional area of the through-hole) is determined. The diameter of a virtual perfect circle having the same area as the determined area is calculated. The calculated values represent the pore diameters of the through-holes of the expanded beads. The above measurement was carried out for 50 expanded beads, and the arithmetic average is the average pore diameter d of the expanded beads.

The average outer diameter D of the expanded beads is determined as follows. First, the expanded beads are cut perpendicular to the penetrating direction of the through-holes at the position where the area of the cut surface is the largest. A photograph of the cut surface of the obtained expanded beads was taken. The area of the expanded beads in the photograph (i.e., the cross-sectional area of the expanded beads including the cross-sectional area of the through-holes) was determined. The diameter of a virtual perfect circle having the same area as the determined area is calculated. The calculated value represents the outer diameter of the expanded beads. The above measurement was carried out on 50 randomly selected expanded beads, and the arithmetic average value was the average outer diameter D of the expanded beads.

It is preferred that the expanded beads of the invention have an average wall thickness t of from 0.8 to 2 mm. When the average wall thickness t is within this range, since the wall thickness of the expanded beads is thick, the expanded beads are less likely to be crushed by an external force. Therefore, the expanded beads located on the surface of the molded article are less likely to be chopped (shredded). For this reason, the average wall thickness t of the expanded beads 2 is more preferably 0.9 to 1.5 mm, still more preferably 1.0 to 1.4 mm.

Conventionally, in the case of thick expanded beads, the pore diameter of the through-holes tends to become small, so that it tends to be difficult to obtain a molded article having a high porosity. According to the expanded beads of the present invention, even if the expanded beads are thick, it is possible to obtain a molded article having a high porosity in a wide molding pressure range.

The average wall thickness t of the expanded beads is determined by the following formula:

t=(D-d)/2

wherein D and D are as defined above.

It is also preferred that the ratio t/d of the average wall thickness t to the average pore diameter d of the through-hole is from 0.4 to 1, more preferably from 0.6 to 0.9. When the ratio t/d is within this range, the difference in compression characteristics of the expanded beads between the penetrating direction of the through-holes and the direction perpendicular to the through-holes becomes small, so that an expanded bead molded article having uniform compression characteristics as a whole can be obtained.

It is further preferred that the average length L of the expanded beads is from 2 to 7 mm. The average length L of the expanded beads is calculated as follows. Fifty expanded beads were randomly selected and each measured for the maximum length of its through-hole in the penetration direction using a caliper. The average length L is the arithmetic mean of the 50 measurements obtained. It is further preferable that the ratio L/D of the average length L to the average outer diameter D of the expanded beads 2 is 0.5 to 2, more preferably 1 to 1.5, because the filling efficiency into the mold at the time of in-mold molding is excellent so that the fusion bondability between the expanded beads is excellent.

Incidentally, even when the expanded beads are formed into an expanded bead molded article, the average pore diameter D, the average outer diameter D, and the average wall thickness t of the expanded beads can be determined in the same manner as the measurement method of the expanded beads. Specifically, the expanded beads forming the expanded bead molded article were sampled, and a photograph of a cross section in a direction perpendicular to the through-hole was taken. The necessary measurements can be made from the photograph using image analysis software or the like.

The bulk density and average cell diameter of the expanded beads of the present invention will be described below. In the present invention, the bulk density of the expanded beads is preferably 15 to 50kg/m ³ More preferably 20 is 40kg/m ³ This is because it is easy to achieve weight reduction and rigidity of the molded article.

The bulk density was measured as follows. Expanded beads were randomly selected from a given group of expanded beads and placed in a measuring cylinder having a volume of 1L so that a large amount of expanded beads were accommodated therein up to the 1L scale in a naturally settled state while static electricity was removed. Next, the weight of the contained expanded beads was measured. Bulk Density of expanded beads (kg/m) ³ ) Is calculated by dividing the weight (g) of the expanded beads by the contained volume (1L) and converting the units appropriately. The measurements were carried out at 23 ℃ and 50% relative humidity at atmospheric pressure.

From the viewpoint of excellent moldability, rigidity, dimensional stability and the like, it is preferred that the expanded beads of the present invention have an average cell diameter of from 50 to 900 μm. For this reason, the average cell diameter is more preferably 80 to 500 micrometers, still more preferably 100 to 250 micrometers.

As used herein, the average cell diameter of the expanded beads is measured in the following manner. The expanded beads were bisected and the cross-section was photographed using a microscope so that the entire cross-section could be seen. A straight line is drawn on the obtained photograph so that the cross section is almost bisected. And a value obtained by dividing the length of a line segment thereof from the periphery of the expanded bead to the opposite periphery thereof (excluding the through-hole portion) by the number of all cells intersecting the line segment is defined as an average cell diameter of the expanded bead. The above operation was performed on 20 expanded beads selected at random, and the value obtained by arithmetically averaging the average cell diameters of the 20 expanded beads was defined as the average cell diameter of the expanded beads.

The method for producing the expanded beads of the present invention will be described below. The expanded bead of the present invention can be produced by preparing multilayer resin particles (or pellets) each having a core layer and a covering layer having a columnar shape, and then expanding the obtained columnar layers of the multilayer resin particles. In this case, by making the thickness of the covering layer of the multilayer resin particles smaller than the average cell diameter of the above-mentioned expanded beads, the expansion of the covering layer in the expansion step of the multilayer resin particles can be suppressed.

The multilayer resin beads can be produced by a known method, for example, a method in which a resin particle production method using a die having a nozzle shape similar to the cross-sectional shape shown in fig. 2 and 3 of the present specification, as described in japanese unexamined patent publication No. H08-108441, is combined with a production method of resin beads having a core layer and a cladding layer, in which a coextrusion method is used, as described in japanese unexamined patent publication No. S41-16125, japanese examined patent publication No. S43-23858b, jp44-29522B, japanese examined patent publication No. S60-185816A, and the like.

For example, the multilayer resin particle is produced in the following manner. An extruder for forming the core layer and an extruder for forming the cover layer are used. The outlets of both extruders were connected to a coextrusion die. The desired resin and the additives to be blended as needed are melted and kneaded in an extruder that forms the core layer, and the desired resin and the additives to be used as needed are also melted and kneaded in an extruder that forms the cover layer. The respective melt-kneaded materials are combined in a die mounted at the front end of the extruder to form a resin having a multilayer structure composed of a core layer and a covering layer covering the outer surface of the core layer, and then the resin is coextruded into a strand from a die equipped with a head having a desired cross-sectional shape. The strands were cut with a pelletizer, thereby producing multilayer resin pellets each having a tubular shape with a predetermined weight.

As the shape of the multilayer resin particle used in the present invention, a tubular shape such as a columnar shape, an elliptical tubular shape, a rectangular tubular shape, or a conjugate shape of a plurality of tubes may be mentioned. The expanded beads obtained by expanding such multilayer resin particles have a shape generally corresponding to the shape of the resin particles before expansion.

The average weight of each of the multilayered resin particles is preferably 0.05 mg to 10.0 mg, particularly preferably 0.1 mg to 5.0 mg. With respect to the average weight of each of the expanded beads, it is preferable that the average weight of each of the expanded beads is 0.05 to 10.0 mg, particularly 0.1 to 5.0 mg in order to improve the filling efficiency into the mold and the fusion property between the expanded beads at the time of in-mold molding. The average weight of each bead of the expanded beads can be controlled by adjusting the average weight of each particle of the multilayer resin particles from which the expanded beads are produced to the average weight of each bead of the targeted expanded beads.

In the multilayer resin particle of the present invention, the weight ratio of the core layer to the covering layer (core layer: covering layer) is preferably 99.5:0.5 to 75, more preferably 98 to 80, still more preferably 96 to 10. When the weight ratio of the covering layer of the multilayer resin particles is within the above range, the melting property of the expanded beads obtained at the time of in-mold molding is ensured, so that the mechanical properties of the expanded bead molded article obtained become good.

As for the thickness of the covering layer of the multilayered resin particle of the present invention, it is preferable that the thickness is thin because the formation of bubbles in the covering layer is prevented when the multilayered resin particle is expanded, so that the mechanical properties of the expanded bead molded article finally obtained are improved. On the other hand, it is desirable to set the lower limit thereof in view of the effect of improving the melting property of the obtained expanded beads. Specifically, the thickness of the cover layer of the multilayered resin particle is preferably 1 to 50 micrometers, more preferably 2 to 20 micrometers, still more preferably 3 to 10 micrometers.

The thickness of the covering layer of the multilayer resin particles was measured in the following manner. The multilayer resin particle was bisected so that the entire periphery of the obtained cross section was surrounded by the covering layer. The photograph is measured, wherein the cross section is enlarged and taken with a microscope so that the entire cross section can be seen. Specifically, on the obtained photograph, a straight line is drawn so that the cross section is almost bisected, and another straight line perpendicular to the straight line and passing through the central portion of the resin particle is drawn. The length of four portions of these straight lines through the cover layer was measured. The arithmetic mean of the measured lengths represents the thickness of the covering layer of the multilayer resin particles. The above operation is performed on 10 randomly selected multilayer resin particles, and the value obtained by arithmetically averaging the thicknesses of the covering layers of the 10 multilayer resin particles is the thickness of the covering layer of the multilayer resin particle in this specification. The thickness of the covering layer of the expanded beads can also be measured by a similar method. When it is difficult to determine the thickness of the covering layer of the multilayer resin particles or expanded beads, the measurement can be performed by using the multilayer resin particles in which a colorant has been added in advance to the resin constituting the covering layer thereof.

The expanded beads of the present invention can be produced by a method comprising dispersing the above-mentioned multilayer resin particles composed of the core layer and the covering layer in an aqueous medium (usually water) in a pressurizable closed vessel (e.g., autoclave), adding a dispersing agent thereto, injecting a required amount of a foaming agent thereinto, impregnating the multilayer resin particles with the foaming agent with stirring at high temperature and high pressure to form expandable multilayer resin particles, and releasing the expandable multilayer resin particles together with the aqueous medium into a region having a lower pressure than the internal pressure of the vessel (usually under atmospheric pressure) to foam and expand the particles (this method is hereinafter referred to as a dispersion medium release foaming method).

The method for obtaining the expanded beads of the present invention is not limited to the dispersion medium releasing expansion method. For example, it is possible to employ a method comprising foaming and expanding resin particles of a base material composed of the resin composition (a) for forming the core layer to obtain expanded beads, and coating the obtained expanded beads with a resin powder formed of the resin (b).

Further, in the case where expanded beads having a particularly low apparent density (high expansion ratio) are to be obtained, such low apparent density expanded beads may be obtained by a method comprising aging the expanded beads obtained by the above-described method under atmospheric pressure in a manner usually employed, then placing them in a pressurizable container, subjecting them to a pressurization treatment by feeding a gas such as air under pressure into the container, thereby increasing the internal pressure of the expanded beads, taking out the obtained expanded beads from the container, and heating them with steam or hot air to expand them again (this method is hereinafter referred to as a two-stage expansion method).

The blowing agent is preferably a physical blowing agent. The physical blowing agent is not particularly limited, but may be, for example, organic blowing agents such as aliphatic hydrocarbons, e.g., n-butane, isobutane, mixtures thereof, n-pentane, isopentane and n-hexane and halogenated hydrocarbons, e.g., ethyl chloride, 2,3,3,3-tetrafluoro-1-propene and trans-1,3,3,3-tetrafluoro-1-propene; and inorganic foaming agents such as carbon dioxide, nitrogen, air and water. These may be used alone or in admixture of two or more. When an organic physical blowing agent or a combination of an inorganic physical blowing agent and an organic physical blowing agent is used, it is preferable to use n-butane, isobutane, n-pentane, isopentane as the organic physical blowing agent because of their excellent compatibility and foamability with olefin resins.

Among these blowing agents, it is preferable to use a blowing agent containing an inorganic physical blowing agent such as carbon dioxide, nitrogen and air, more preferably carbon dioxide, as a main component. As used herein, "inorganic physical blowing agent as a main ingredient" is intended to mean that the inorganic physical blowing agent is contained in an amount of 50mol% or more, preferably 70mol% or more, still more preferably 90mol% or more in 100mol% of the total physical blowing agents.

The amount of the physical blowing agent is appropriately selected depending on the type of the propylene-based resin, the type of the blowing agent, the apparent density of the desired expanded beads, and the like, but is not particularly limited. When carbon dioxide is used as the physical blowing agent, for example, it is used in an amount of 0.1 to 30 parts by weight, preferably 0.5 to 15 parts by weight, still more preferably 1 to 10 parts by weight, based on 100 parts by weight of the propylene-based resin.

As the dispersant, water-insoluble inorganic substances such as alumina, tricalcium phosphate, magnesium pyrophosphate, zinc oxide, kaolin and mica, and water-soluble polymeric protective colloids such as polyvinylpyrrolidone, polyvinyl alcohol and methylcellulose can be used. Further, anionic surfactants such as sodium dodecylbenzenesulfonate and sodium alkylsulfonate may also be used.

When the expandable multilayer resin particles are released from the closed vessel together with the dispersion medium, it is preferable to perform pressure control by applying pressure (back pressure) to the closed vessel with carbon dioxide, nitrogen or the like so that the pressure in the opened vessel is constant or gradually increased.

In the foaming step, a heating rate of 0.5 ℃/min to 5 ℃/min is generally employed.

The method for producing expanded beads of the present invention includes a step of forming a so-called high temperature peak in which the multi-layered resin particles are heat-treated by being held in a dispersion medium within a specific temperature range before expanding the expandable multi-layered resin particles. The heat treatment may be performed at any point in time before, during and after impregnation of the blowing agent, and may also be performed at these two or more points in time. By such heat treatment, expanded beads having a crystal structure showing a main endothermic melting peak (intrinsic peak) derived from the crystals intrinsic to the polypropylene resin composition (a) and a melting peak (high-temperature peak) located on a higher temperature side than the intrinsic peak can be obtained. The heat treatment is performed, for example, as follows. The resin particles are held at a temperature close to the melting point (Tm) of the polypropylene-based resin composition (a) as the base resin of the multilayered resin particles, more specifically, at any temperature within a range of not lower than a temperature 15 ℃ lower than the melting point (Tm-15 ℃) and a temperature lower than the melting end temperature (Te), for a sufficient time (preferably about 5 to 60 minutes). Thereby, a part or all of the crystals inherent in the polypropylene-based resin composition (a) are melted and a part of the melted crystals are recrystallized to produce thick sheet-like high potential crystals. By subsequently foaming the expandable multilayer resin particles having high potential crystals at the foaming temperature, expanded beads having a crystal structure with high potential crystals that have crystals formed by crystallization of molten crystals (intrinsic crystals) due to cooling during foaming and show a peak on the high temperature side are obtained.

The expanded beads can be obtained by subsequently releasing the expandable resin particles into a low-pressure atmosphere. The temperature of the contents in the closed container at the time of discharging the expandable resin particles together with the aqueous dispersion medium from the closed container, that is, the temperature at the time of expanding the expandable resin particles (foaming temperature) is preferably in the range from a temperature (Tm-15 ℃) lower by 15 ℃ than the melting point (Tm) of the polypropylene-based resin composition (a) to a temperature (Te +10 ℃) higher by 10 ℃ than the melting completion temperature. The difference between the pressure in the closed vessel and the pressure in the discharge atmosphere is preferably 1.0MPa to 7.0MPa, more preferably 1.5MPa to 5.0MPa.

In the dispersion medium releasing foaming method, the ratio (inherent peak heating value/high temperature peak heating value) can be controlled to 3.5 or more by adjusting the foaming temperature to a high value, increasing the injection amount of the foaming agent, or the like.

The temperature range adjusted when the above resin particles are foamed is a suitable temperature range when an inorganic physical foaming agent is used as a foaming agent. When an organic physical blowing agent is additionally used in combination, the appropriate temperature range tends to shift toward a lower side than the above temperature range due to the plasticizing effect of the organic physical blowing agent on the base resin depending on the type and amount thereof.

The polypropylene-based resin expanded bead molded article of the present invention will be described below. The expanded bead molded article is obtained by in-mold molding of propylene-based resin expanded beads, and has interconnected pores.

A schematic diagram of an example of the surface state of the expanded bead molded article of the present invention is shown in fig. 5. Fig. 5 shows an expanded bead molded article 1 having a plurality of expanded beads 12 and interconnected pores 6. Thus, the propylene-based resin expanded bead molded article having interconnected pores of the present invention has the pores 6 as shown in fig. 5 and is obtained by in-mold molding of propylene-based resin expanded beads.

As a method for producing an expanded bead molded article by molding the expanded beads of the present invention in a mold, a known in-mold molding method can be mentioned.

One example is a cracking molding process, in which expanded beads are subjected to in-mold molding using a pair of molds. The expanded beads are filled in the mold cavity under atmospheric pressure or reduced pressure, and the mold is closed such that the volume of the mold cavity is reduced by 5 to 50 vol% to compress the expanded beads. A heating medium such as steam is then fed into the mold to heat and melt-bond the expanded beads (for example, japanese examined patent publication No. S46-38359). The molding may also be performed by a pressure molding method in which the expanded beads are subjected to a pressure treatment with a pressurized gas such as air to increase the pressure inside the expanded beads. The resulting expanded beads are then filled in the mold cavity at atmospheric pressure or reduced pressure and the mold is closed. A heating medium such as steam is then fed into the mold to heat and melt-bond the expanded beads (for example, japanese examined patent publication No. S51-22951). Further, the molding may be performed by a compression filling molding method which includes pressurizing the mold cavity to a pressure higher than atmospheric pressure with compressed gas, then filling the expanded beads in the mold cavity while pressurizing them to the pressure or a pressure higher than the pressure, and then supplying a heating medium such as steam into the mold to heat and melt-bond the expanded beads (for example, japanese examined patent publication No. H4-46217). In addition, molding may be performed by an ambient pressure filling molding method in which expanded beads are filled in a mold cavity defined by a pair of molds at ambient pressure. And then a heating medium such as steam is supplied to heat and melt-bond the expanded beads (for example, japanese examined patent publication No. H6-49795). A method in which the above methods are combined may also be employed (for example, japanese examined patent publication No. H6-22919).

It is preferable that the expanded bead molding produced by in-mold molding of the expanded beads of the present invention has a density of 15 to 50kg/m ³ More preferably 20 to 40kg/m ³ Density within the range to achieve light weight and rigidity.

Density (kg/m) of expanded bead molded article ³ ) It may be calculated by dividing the weight (g) of the molded article by the volume (L) determined from the outer dimensions of the molded article and converting the units appropriately.

The expanded bead molding of the present invention preferably has a porosity of 20% or more. When the porosity is 20% or more, the expanded bead-molded article exhibits excellent sound absorption, air permeability, water permeability, damping properties, and the like. Further, when the molded article is used as an automobile part, an effect of improving adhesion to polyurethane foam is obtainable. For this reason, the porosity of the molded article is more preferably 22% or more, still more preferably 24% or more. The upper limit is generally 40%, preferably 35%.

The porosity of the expanded bead molding was determined as follows. A test piece in a rectangular parallelepiped shape cut from a molded article of expanded beads except a skin layer was immersed in a volume containing alcohol, and a true volume Vt (cm) of the test piece was measured from a liquid level rise of the alcohol ³ ). Further, apparent volume Va (cm) ³ ) Determined by the outer dimensions (length x width x height) of the test piece. From the obtained true volume Vt and apparent volume Va, the porosity of the expanded bead molded article was calculated according to the following formula.

Porosity (%) = [ (Va-Vt)/Va ]. Times.100

It is preferable that the expanded bead molded article of the present invention has a 50% compressive stress of 120 to 300kPa, more preferably 150 to 250kPa, still more preferably 185 to 240kPa, because of an excellent balance between the rigidity and the cushioning property of the molded article. When the 50% compressive stress is within the above range, it is likely that the generation of frictional noise is suppressed when the molded article is used as an automobile member. The 50% compressive stress is an index for measuring the rigidity of the foamed article.

The 50% compressive stress of the expanded bead molded article was measured as follows.

A test piece 50 mm long × 50 mm wide × 25 mm thick was cut out from the expanded bead molded article in such a manner that the skin layer was excluded therefrom and the compressive stress (MPa) at 50% strain was measured when compressed at a rate of 10mm/min based on JIS K6767 (1999).

The expanded beads of the present invention are excellent in melting property between each other, allowing in-mold molding in a wider range of molding vapor pressure, as compared with conventional expanded beads. The obtained expanded bead molded article has high porosity, has desirable properties such as air permeability, water permeability, sound absorption, sound deadening properties, vibration insulating properties, etc. due to the interconnected pores thereof, and is excellent in mechanical properties. For this reason, the article can be suitably used as a drainage material, a wall material in a building, an interior material for a motor vehicle, a cushioning material, or the like. Further, the article is suitable for use as an automotive member such as an automotive seat member due to its excellent adhesion to polyurethane foam.

The embodiment is as follows:

the present invention will be described in more detail below by way of examples. However, the present invention is not limited to these examples in any way.

The resins shown in table 1 were used in the examples and comparative examples.

The melting points and heats of fusion in table 1 were measured as follows.

According to the heat flow type differential scanning calorimetry method described in JIS K7121 (2012), a DSC curve was obtained by heating a test piece of 2 mg of a particulate base resin from 30 ℃ to 200 ℃ at a heating rate of 10 ℃/min, then cooling to 30 ℃ at a cooling rate of 10 ℃/min, and heating again from 30 ℃ to 200 ℃ at a heating rate of 10 ℃/min. The peak top temperature of the endothermic curve is defined as the melting point of the resin. Further, the area of the endothermic peak is defined as the heat of fusion of the resin. As the measurement apparatus, a heat flow type differential scanning calorimetry apparatus (DSC Q1000 manufactured by TA Instruments inc.).

The flexural modulus of the resins in table 1 was measured according to JIS K7171 (2008). The resin was hot-pressed at 230 ℃ to prepare a 4 mm sheet, from which a test piece (standard test piece) 80 mm long, 10mm wide and 4 mm thick was cut. The radius R1 of the indenter and the radius R2 of the carrier were both 5 mm, the distance between the fulcrums was 64 mm, and the test speed was 2 mm/min.

The MFR in table 1 was measured by the method described previously. Specifically, according to JIS K7210-1 (2014), the measurement was performed under the conditions including the temperature of 230 ℃ and the load of 2.16kg in the case of the polypropylene-based resin and the conditions including the temperature of 190 ℃ and the load of 2.16kg in the case of the polyethylene-based resin.

Cell controlling agent masterbatch:

a cell control agent master batch prepared by blending 90wt% of a polypropylene-based resin with 10wt% of zinc borate was used.

In examples and comparative examples, multilayer resin particles were prepared using the following apparatus. An extruder having an inner diameter of 100 mm and an L/D of 32 was used as an extruder for forming the foamed core layer, and an extruder having an inner diameter of 25 mm and an L/D of 32 was used as an extruder for forming the covering layer. The outlets of the extruder forming the foamed core layer and the extruder forming the cover layer are connected to an annular die for coextrusion so that the respective resin melts can be laminated in the coextrusion annular die.

Examples 1 to 5 and comparative examples 1 to 5

Production of multilayer resin particles:

the resin for forming a foamed core layer and the blowing agent master batch shown in tables 2 and 3 were supplied to the first extruder in a ratio in which the master batch was 5 parts by weight per 100 parts by weight of the resin for forming a core layer, and melt-kneaded therein to obtain a resin melt for forming a foamed core layer. Meanwhile, the resins for forming the covering layer shown in tables 2 and 3 were supplied to a third extruder and melt-kneaded therein to obtain a resin melt for forming the covering layer. The resin melt for forming the foamed core layer and the resin melt for forming the cover layer were introduced into a co-extrusion die and laminated so that the resin melt for forming the cover layer surrounded the resin melt for forming the foamed core layer so that the weight ratio of the cover layer was as shown in tables 2 and 3. The laminate was coextruded into a multilayer strand from a small orifice of a coextrusion die connected to the front end of the extruder, wherein a cover layer was provided on the outer periphery of the tubular core layer having the through-holes. The strand was then cooled with water, cut to a weight of about 1.5 mg with a pelletizer and dried to give multilayer resin pellets. In the above production of the multilayer resin particles, the pore diameter of the die was adjusted so that the average outer diameter D0, the average pore diameter D0 and the average wall thickness t0 of the resin particles had the values shown in tables 2 and 3. The average outer diameter D0, the average pore diameter D0 and the average wall thickness t0 of the resin particles were measured in the same manner as the above-mentioned average outer diameter D, average pore diameter D and average wall thickness t of the expanded beads.

Production of expanded beads:

next, using the multilayer resin particles, propylene resin expanded beads were produced.

First, 1kg of the multilayer resin particles obtained as described above was charged into a 5L closed container equipped with a stirrer together with 3L of water as a dispersion medium, and 0.3 parts by weight of kaolin as a dispersant, 0.004 parts by weight (amount of active ingredient) of a surfactant (trade name: NEOGEN S-20F, manufactured by Daiichi Kogyo Seiyaku Co., ltd., sodium alkylbenzenesulfonate), and carbon dioxide as a foaming agent were injected therein so that the pressure (CO) was increased ₂ Injection pressure) are as shown in tables 4 and 5。

Then, the temperature of the dispersion medium was increased to "foaming temperature-5 ℃" shown in tables 4 and 5 with stirring, and kept at that temperature for 15 minutes. Then, the temperature was raised to the foaming temperature shown in tables 4 and 5 and kept at the temperature for 15 minutes, and subsequently, the contents were released to atmospheric pressure while applying back pressure shown in tables 4 and 5 with carbon dioxide to obtain substantially columnar foamed beads having through-holes. The physical properties of the expanded beads obtained are shown in tables 4 and 5.

The amounts (parts by weight) of the dispersant and the surfactant added are based on 100 parts by weight of the amount of the propylene-based resin particles.

The expanded beads obtained had a tubular shape as shown in FIG. 1. Observation of the cross section thereof with an optical microscope revealed that the resin constituting the foam core layer foamed well to form a closed cell structure, whereas the resin constituting the cover layer did not foam.

The average inner diameter D of the through-holes and the average outer diameter D of the expanded beads were measured by the aforementioned methods. In the measurement of the average inner diameter d of the through-hole of the expanded bead, the expanded bead is first cut at a position where the area of the cut surface is largest in the direction perpendicular to the penetrating direction of the through-hole. A photograph of the obtained cut surface of the expanded bead was taken. The area of the through-hole portion in the photograph (i.e., the cross-sectional area of the through-hole) is determined. The diameter of a virtual perfect circle having the same area as the determined area is calculated. The calculated value represents the diameter of the through-hole of the expanded beads. The above measurement was carried out for 50 expanded beads, and the arithmetic average thereof was taken as the average pore diameter d of the expanded beads.

In the measurement of the average outer diameter D of the expanded beads, the expanded beads are first cut at a position where the area of the cut surface is maximum in a direction perpendicular to the penetration direction of the holes. A photograph of the cut surface of the obtained expanded beads was taken. The area of the expanded beads in the photograph (i.e., the cross-sectional area of the expanded beads including the cross-sectional area of the through-holes) was determined. The diameter of a virtual perfect circle having the same area as the determined area is calculated. The calculated value represents the outer diameter of the expanded beads. The average outer diameter D of the expanded beads was determined as an arithmetic average value of 50 randomly selected expanded beads. The average wall thickness t of the expanded beads was calculated on the basis of (D-D)/2. The results are shown in tables 4 and 5.

The bulk density of the expanded beads was measured by the aforementioned method. Specifically, expanded beads were randomly taken out from the expanded bead group and placed in a measuring cylinder having a volume of 1L, so that a large amount of expanded beads were accumulated to the scale of 1L in a naturally deposited state. The mass W1[ g ] of the accumulated expanded beads was measured]. This operation was performed 5 times for different expanded bead samples. The bulk density of each expanded bead sample was calculated from each measurement using unit conversion. The arithmetic mean of these calculated values was defined as the bulk density (kg/m) of the expanded beads ³ )。

The melting point of the expanded beads was measured by the aforementioned method. Specifically, according to the heat flow type differential scanning calorimetry method described in JIS K7121 (2012), a DSC curve was obtained using 3mg of expanded beads as a test piece, increasing the temperature from 30 ℃ to 200 ℃ at a heating rate of 10 ℃/min, then decreasing to 30 ℃ at a cooling rate of 10 ℃/min, and again increasing from 30 ℃ to 200 ℃ at a heating rate of 10 ℃/min. The peak temperature of the endothermic peak was determined and taken as the melting point of the expanded beads. As the measurement apparatus, a heat flow type differential scanning calorimetry apparatus (DSC Q1000 manufactured by TA Instruments inc.).

The high-temperature peak heating value and the inherent peak heating value (low-temperature peak heating value) of the expanded beads were measured by the measurement method according to JIS K7122 (2012) as described previously. First, about 3mg of expanded beads were sampled and measured using a heat flow type differential scanning calorimetry apparatus to obtain a DSC curve in which the temperature was increased from 30 ℃ to 200 ℃ at 10 ℃/min. In the following description, a peak inherent to the resin is referred to as a, and a high-temperature peak appearing on the higher temperature side is referred to as B. The expanded beads are plotted on a line (α - β) connecting a point α on the DSC curve at 80 ℃ and a point β on the DSC curve at the final temperature T of melting. The final temperature T of melting is a crossing point at which the DSC curve on the high temperature side of the high temperature peak B satisfies the baseline. Next, a line is drawn parallel to the ordinate and passing through a point γ on the DSC curve of the bottom of the concave portion between the intrinsic peak a and the high-temperature peak B. This point, at which this line intersects the line (α - β), is called σ.

The area of the high temperature peak B is an area bounded by a curve of the high temperature peak B, a line segment (σ - β), and a line segment (γ - σ), and corresponds to the high temperature peak calorific value Δ Hh. The area of the intrinsic peak is an area bounded by a curve of the resin intrinsic peak a, a line segment (α - β), and a line segment (γ - σ), and corresponds to the intrinsic peak calorific value Δ Hm. The total heat of fusion of the expanded beads was calculated by adding the high temperature peak heating value and the main endothermic peak heating value (intrinsic peak heating value).

As the measurement apparatus, a heat flow type differential scanning calorimetry apparatus (DSC Q1000 manufactured by TA Instruments inc.).

Production of expanded bead moldings:

the obtained expanded beads were used for in-mold molding of an expanded bead molded article.

First, expanded beads were packed in a flat mold having a length of 300 mm, a width of 250 mm and a thickness of 60 mm. The flat-plate-like expanded bead molded article is obtained by in-mold molding by a pressure molding method using steam as a heating medium.

The heating method during in-mold molding is as follows. The preheating is performed by supplying steam for 5 seconds while keeping the drain valves on both sides of the mold in an open state. Then, unidirectional flow heating was performed at a pressure 0.04MPa (G) lower than the molding vapor pressure shown in tables 6 and 7. Further, the counter-unidirectional flow heating was performed at a pressure 0.02MPa (G) lower than that used for the complete heating. Thereafter, the molded vapor pressure [ MPa (G) in tables 5 to 7; gauge pressure ], heating with steam.

After the heating was completed, the pressure was released and water cooling was performed until the value of the surface pressure gauge attached to the inner surface of the mold was reduced to 0.04MPa (G). The mold is then opened. The molded article was taken out of the mold cavity, aged in an oven at 60 ℃ for 12 hours, and then slowly cooled to obtain an expanded bead molded article. The physical properties of the molded articles obtained are shown in tables 6 and 7. Thus, a flat-plate-like expanded bead molded article having a thickness of 60 mm and having interconnected pores was obtained. Tables 6 and 7 show the physical properties and melt bondability evaluation of the expanded bead moldings.

In tables 6 and 7, "vapor pressure range 1" is a moldable range. Expanded bead moldings were prepared at steam pressures varying between 0.08 and 0.30MPa (G) at 0.02MPa (G) intervals. The prepared molded articles were evaluated for both melt bondability and recoverability according to the criteria described below, and those molded articles evaluated as "a" in both items were considered acceptable. "vapor pressure range 1" refers to the range of vapor pressures that provide such acceptable articles. A broader range of expanded beads having a moldable vapor pressure from a lower value to an upper value is considered to have a broader moldable range and better mold-moldability. Further, the expanded beads exhibiting the lower limit value of the vapor pressure that can be molded are considered to be better able to be molded at a lower pressure and to have excellent in-mold moldability.

In tables 6 and 7, "vapor pressure range 2" is a range capable of providing a molded article with high porosity. The expanded bead moldings prepared as above were further measured for their porosity. "vapor pressure range 2" means a range of vapor pressures within the above moldable range and providing an expanded bead molded article having a porosity of 20% or more.

In tables 6 and 7, physical properties and evaluations were determined as follows.

Density of expanded bead molded article:

the density of the expanded bead molded article (with skin) was calculated by dividing the weight (g) of the molded article with skin obtained by in-mold molding by the volume (L) determined from the outer dimensions of the molded article. Measurements were carried out on expanded bead moldings aged at 23 ℃ and 50% relative humidity for 48 hours under atmospheric pressure.

The porosity of the expanded bead molded article was determined by the aforementioned method. Specifically, a test piece in the shape of a rectangular parallelepiped cut out from a molded article of expanded beads except for the skin layer and having a length of 20 mm, a width of 100 mm and a thickness of 20 mm was immersed in a volume containing alcohol, and a true volume Vt (cm) of the test piece was measured from a rise of a liquid surface of the alcohol ³ ). Next, the apparent volume Va (cm) ³ ) Determined by the external dimensions (length, width, height) of the test piece. From the obtained true volume Vt and apparent volume Va, the porosity of the expanded bead molded article was determined according to the above formula (2).

Percent shrinkage:

the shrinkage percentage of the expanded bead molded article was measured as follows.

The shrinkage percentage [% ] of the expanded bead molded article was determined by:

(300, [ mm ]/500, [ mm ]. Times.100, the length of the long side of the molded article.

"300[ mm ]" is the dimension of the long side of the molding die. "the long side length [ mm ] of the molded article" is a measurement value obtained by a method in which the expanded bead molded articles obtained in examples and comparative examples have been aged at 80 ℃ in the atmosphere for 12 hours, then slowly cooled, and then further aged at 23 ℃ in the atmosphere for 6 hours, and then the long side lengths thereof are measured.

Evaluation of melt bondability of expanded beads:

the expanded bead molded article is bent and broken. The fracture surface thereof was observed to determine the material fracture ratio (%) obtained by dividing the number of broken expanded beads present on the fracture surface by the number of whole expanded beads present on the fracture surface. Evaluation was performed according to the following criteria.

A: when the expanded bead molded article is broken, the material breakage rate of the expanded beads is 90% or more;

b: when the expanded bead molded article is broken, the material failure rate of the expanded beads is less than 90;

evaluation of recoverability of expanded beads:

the in-mold molding was performed using a flat plate-shaped mold having a length of 300 mm, a width of 250 mm and a thickness of 60 mm. The thickness of the obtained expanded bead-molded article was measured at the vicinity of the four corners (10 mm from the center of the angle) and at the central portion (both longitudinally and transversely bisected portions). Next, a ratio (%) of the thickness of the central portion based on the thickness of the thickest portion of the four corners is calculated. The evaluation a is assigned when the ratio is 90% or more, and the evaluation B is assigned when the ratio is less than 90%.

Compressive stress at 50% strain:

a test piece having a length of 50 mm, a width of 50 mm and a thickness of 25 mm was cut out from the central portion of the expanded bead molded article except for the skin layer thereof. A compression test was conducted at a compression rate of 10mm/min in accordance with JIS K6767 (1999) to measure 50% compressive stress. The density of the test piece used for this measurement is shown in the table as the density of the expanded bead molded article (without skin). The density of the expanded bead-molded article (without skin) was measured in the same manner as the density of the expanded bead-molded article except that the test piece used was cut from the molded article in such a manner that the skin was removed.

With the expanded beads of the present invention used in examples, expanded bead molded articles having a high porosity can be molded over a wide range of molding vapor pressures. In particular, the expanded beads of the present invention allow in-mold molding at a molding vapor pressure as low as about 0.12MPa (G). Further, even under high pressure conditions including a molding vapor pressure of 0.20MPa, an expanded bead molded article having a porosity of 20% or more can be obtained. The molded article obtained is light in weight and has excellent rigidity.

Further, in example 3 in which the content of the high melting point resin PP2 was 5% by weight, the lower limit of the moldable water vapor pressure showed a particularly low value of 0.10MPa (G).

Comparative example 1 is an example in which the foamed core layer is formed from the low-melting resin PP1 without containing the high-melting resin PP2 and in which the ratio of the intrinsic peak heat value to the high-temperature peak heat value is made low. The molding water vapor pressure is high and the range of moldability is narrow. Further, the compressive stress at 50% strain is slightly lower.

Comparative example 2 is an example in which the foam core layer is formed of the low melting point resin PP1 without containing the high melting point resin PP2 and in which the ratio of the intrinsic peak calorific value to the high temperature (peak) calorific value is set to the same level as in example 1. Although low pressure molding is possible, the compressive stress at 50% strain is somewhat lower. In addition, the porosity decreases under high molding vapor pressure conditions.

Comparative example 3 is an example in which a large amount of 40wt% of the high melting point resin PP2 was used. The effect of the high melting point resin PP2 is so great that a high molding vapor pressure is required.

Comparative example 4 is an example in which the resin 2 having a low melting point was used instead of the low melting point resin PP 1. Under high molding vapor pressure conditions, porosity decreases.

Comparative example 5 is an example in which the resin composition was the same as in example 1 and in which the ratio of the intrinsic peak heating value to the high-temperature peak heating value was controlled to be small by increasing the foaming temperature. The molding vapor pressure was high, so that the compressive stress at 50% strain of the expanded bead molded article obtained was slightly lower. In comparative examples 1,3 and 5, the lower limit of the "vapor pressure range 2" was poor in high and low pressure moldability, and thus "vapor pressure range 2" was not evaluated.

Description of the symbols:

1: expanded bead molded article

2: expanded beads

3: foamed core layer

4: through hole

5: cover layer

6: pores of

12: and (3) foaming the beads.

Claims

1. A foamed bead having through-holes, the foamed bead including a foamed core layer defining the through-holes therein and including a polypropylene-based resin composition, and a cover layer covering the foamed core layer and including a polyolefin-based resin,

wherein the first DSC curve has a main endothermic peak inherent to the polypropylene-based resin composition and a high temperature side endothermic peak located on the higher temperature side of the main endothermic peak,

wherein the high-temperature side endothermic peak has a heat of fusion of 12 to 20J/g, and

2. The polypropylene-based resin expanded bead according to claim 1, wherein the melting point of the polypropylene-based resin PP1 is higher than 140 ℃ and not higher than 145 ℃, and the melting point of the polypropylene-based resin PP2 is not lower than 150 ℃ and not higher than 155 ℃.

3. The polypropylene-based resin expanded bead according to claim 1 or 2, wherein the polypropylene-based resin PP2 has a Melt Flow Rate (MFR) at 230 ℃ and under a load of 2.16kg of from 2 to 18g/10 min.

4. The polypropylene-based resin expanded bead according to any one of claims 1 to 3, wherein the polypropylene-based resin PP1 and the polypropylene-based resin PP2 are each a polypropylene-based resin obtained by polymerization using a Ziegler-Natta catalyst.

5. The polypropylene-based resin expanded bead according to any one of claims 1 to 4Pellets having 15 to 50kg/cm ³ The bulk density of (2).

6. The polypropylene-based resin expanded bead according to any one of claims 1 to 5, which has an average outer diameter D [ mm ], an average pore diameter D [ mm ] of through-holes and an average wall thickness t [ mm ] defined as (D-D)/2,

wherein t is 0.8 to 2mm and t/d is 0.4 to 1.

7. A polypropylene-based resin expanded bead molded article comprising a plurality of expanded beads according to any one of claims 1 to 6, the expanded beads being fusion-bonded to each other, the expanded bead molded article being formed with interconnected pores and having a porosity of 20% or more.